Cholesterol is a compound critical for life, but elevated serum cholesterol levels increase the risk of heart attack and stroke. The liver is a critical organ for cholesterol homeostasis. In the liver, cholesterol is synthesized and converted to bile acids. The cytochrome P450 monooxygenase family of enzymes plays a prominent role in cholesterol metabolism in the liver. In HEK293 cells, the heme-binding protein progesterone receptor membrane component 1 (PGRMC1) is a positive regulator of CYP51A1, a cytochrome P450 enzyme required for cholesterol synthesis. PGRMC1 knockdown cells have impaired cholesterol synthesis and accumulate the intermediate lanosterol (which is known to inhibit HMG-CoA reductase). PGRMC1 also binds another cytochrome P450 enzyme, CYP7A1, required for bile acid synthesis. Based on these findings, I hypothesize that PGRMC1 is an important regulator of cholesterol metabolism and necessary for systemic cholesterol homeostasis in mammals. To test this hypothesis, I will take advantage of the existing Pgrmc1 knockout (Pgrmc1KO) mice generated by our laboratory. In Aim 1, I will test the in vivo role of PGRMC1 in cholesterol synthesis. I will first perform a general histological characterization on the livers of Pgrmc1KO mice and check for markers of liver injury. Then, I will measure hepatic sterol composition, serum cholesterol levels, and serum lipoprotein particle profiles in Pgrmc1KO mice. I will also measure the rate of cholesterol synthesis in Pgrmc1KO primary hepatocytes, and Cyp51A1 enzymatic activity in Pgrmc1KO liver microsomes. In Aim 2, I will test the in vivo role of PGRMC1 in bile acid synthesis. I will measure steady state bile acid levels and hepatic bile acid synthesis rates in Pgrmc1KO mice and Cyp7A1 activity in Pgrmc1KO liver microsomes. These studies will establish the role of PGRMC1 in regulating cholesterol homeostasis. Patients with high serum cholesterol levels are treated with statin drugs (HMG-CoA reductase inhibitors) and bile acid sequestrant drugs. Inhibition of PGRMC1 may prove to be a novel method of inhibiting HMG-CoA reductase by lanosterol accumulation. Alternatively, manipulating PGRMC1 may be a novel method for stimulating bile acid synthesis. This proposal will expand our understanding of how cholesterol homeostasis is maintained in vivo and provide a candidate target for the development of therapeutics for high cholesterol. Treating high cholesterol will reduce the risk of heart attack and stroke in patients. Also, these studies on PGRMC1 will advance our understanding of the cytochrome P450 family of enzymes. P450 enzymes are also important for drug metabolism and have no known post-transcriptional regulators.